Mariko Takahashi and Toshikazu Hasegawa at the University of Tokyo in Japan studied peacocks and peahens in Izu Cactus Park, Shizuoka, from 1995 to 2001.

They judged tail quality in two ways – first by simply measuring tail length, and secondly by taking photos of each male during the tail-fanning display ritual and counting the number of eyespots. Next they examined whether females chose mates with the best-quality tails.

During the seven years of observation, Takahashi’s team observed 268 successful matings. But surprisingly, they found that females mated with poor-quality peacocks as often as with “flashy”, high-quality males.

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They conclude that the peacock’s train is not the object of female sexual preference – contradicting Darwin’s theory of sexual selection.

Negative data

Behavioural ecologist Marion Petrie at the University of Newcastle, UK, has dismissed the study.

“All they have done is fail to find a relationship,” she says. “The authors seem to ignore the fact that three previous independent studies have found relationships between mating success and train morphology. Rather than consider what is unusual about their study, they conclude that peahens in general do not prefer males with elaborate trains.”

Takahashi argues that it is the failure to find a relationship that makes her study so important. “Unfortunately because negative data have been seldom published, they are seldom discussed,” she says.

Because it is “negative data”, Petrie says she doubts she would have been able to get this study published.

Hormonal factor

Takahashi points out that growth of the peacock’s train is dependent on the absence of oestrogen rather than the presence of testosterone. She says this undermines the assumption that the train is a sexual signal.

“Until now, who cared that the peacock’s train was under oestrogen control?” Takahashi says. “We hope our paper will encourage others with [negative] data to publish.”

But another peacock specialist, Adeline Loyau at the University Pierre and Marie Curie, Paris, echoes Petrie’s concerns.

“My major problem is that they didn’t consider the complexity of the signal,” she says. “They only looked at the number of eyespots and train length as a sign of train elaboration. The number of eyespots didn’t correlate with mating, so they concluded that the train signals nothing.”

‘Complex issue’

The peacock’s train is a highly elaborate structure, Loyau adds.

“It’s not just the number of eyespots – it’s the density of spots, it’s the arrangement of patterns, it’s colour – they didn’t talk about the colour at all.”

Louise Barrett at the University of Lethbridge in Alberta, Canada, is an executive editor on Animal Behaviour, the journal that published Takahashi’s study.

“It is perfectly true that the Takahashi study didn’t consider colour,” she says. “But they did consider many other aspects of tail elaboration and they failed to find any effect.”

The arguments against Takahashi’s study wrongly suggest that the latest findings “trump” previous results, says Barrett. “Rather it illustrates that the story is more interesting and complex than we thought. One should never be too complacent and think that a problem has been solved,” she says.